The present invention relates to data processing systems and, more particularly, to data processing systems of the type having a plurality of subsystems linked by a star coupler.
Star couplers are known in the art. Systems employing star couplers or couplers similar to star couplers are described, for example, in the following publications:
Hudson and Thiel, The Star Coupler: A Unique Interconnection Component for Multimode Optical Waveguide Communications Systems, 13 Applied Optics 2450 (November, 1974); PA0 Metcalfe and Boggs, Ethernet: Distributed Packet Switching for Local Computer Networks, 19 Comm. of the ACM 395 (July, 1976); PA0 Rawson and Metcalfe, Fibernet: Multimode Optical Fibers for Local Computer Networks, 26 IEEE Trans. on Comm. 983 (July, 1978).
When used in a data processing system having a plurality of subsystems, a star coupler is typically connected to pairs of transmission lines, with one pair associated with each subsystem and wherein a first transmission line of the pair carries signals away from the subsystem and a second transmission line of the pair carries signals to the subsystem. When any subsystem transmits or generates a signal or message, that message is received by the star coupler from the first transmission line associated with the subsystem and, in turn, is directed or passed to every subsystem by way of each second transmission line, including the second transmission line returning to the subsystem that transmitted the message. This, of course, offers advantages in linking multiple subsystems, since a subsystem transmitting a message receives back the message at the same time that each of the other subsystems receives the message. The transmitting system can check for any transmission errors, without requiring a receiving subsystem to regenerate the message.
There are, of course, problems encountered in a system using a star coupler if two subsystems should transmit simultaneously or nearly simultaneously. In such a case, the signal received by each subsystem is garbled or distorted because of the superimposition of one message on another. Requiring a retransmission in order to correct the problem slows down the operations that are being carried out in the system, since the subsystems having the messages garbled must use processing time later in order to retransmit the messages.